Backupy

- add `--version` flag
- add extension example

- added `--nofollow` flag to copy symbolic links verbatim
- improved error messages
- refactored file operation functions

- minor refactoring and cleanup

- fix file filters
- experimental cold storage support
- use prompt to view file trees with curses instead of flag
- improved error messages
- refactor some code (still some work to be done, naming convention within files is mostly consistent, but all files should eventually follow https://google.github.io/styleguide/pyguide.html#316-naming)